Method for making float glass, implementing device and resulting products

ABSTRACT

The present invention relates to a float process for manufacturing glass sheets, in which molten glass is poured onto a liquid support denser than the glass and then the continuous ribbon which forms is advanced toward the downstream end, this process being characterized in that the thickened edges of the ribbon are trimmed continuously in the forming zone. This process makes it possible to obtain thin glass sheets, such as sheets of film glass, with good flatness, particularly in small plants. The invention also relates to a plant for implementing the process and to the products obtained. FIG.  1.

The present invention relates to a process for manufacturing float glass(or a process for manufacturing flat glass using the float technique orfloat process), this process making it possible to manufacture glasssheets intended notably for the production of glazing. The presentinvention also relates to a plant (or device) for implementing theprocess and to the products obtained.

The manufacture of sheet glass by the float process is carried out inthe known manner by pouring molten glass onto a liquid support (or“bath”) denser than the glass (for example a tin bath) and by advancing,over the surface of the bath, the glass ribbon which forms, before saidribbon, set by cooling, is cut into sheets. In the float plant (or“float”), the glass enters molten at a viscosity of about 10^(3.4)poise, so as to flow over the bath, the temperature at the hot end ofthis bath generally being about 1000–1400° C. (depending on the glassused), and spreads out in order to find its equilibrium thickness. Theforming of the glass, that is to say the operation intended to give theglass ribbon its desired thickness (for example by lateral andlongitudinal stretching, the forming also including, where appropriate,the necking which occurs after the lateral stretching elements), is thencarried out when the glass reaches a viscosity of greater than 10^(3.7)or 10⁴ poise (at temperatures generally below approximately 1150° C.,the viscous force being too low at higher temperatures) and before theglass leaves the bath driven by lift-out rolls downstream of the bathand before it sets (the glass setting at a viscosity of about 10¹³ poiseat the lift-out rolls, the viscosity of the glass leaving the floatbeing, moreover, generally greater than 10¹⁰ poise) and/or before theglass is, where appropriate, annealed.

In a standard process, the lateral stretching of the glass ribbon on thesurface of the bath is carried out by gripping it laterally by top rollspositioned along the bath on each side of the glass ribbon. The plantsusing this process are generally plants with a high output (up to a fewhundred tons of glass per day) and are tailored to the production ofglass sheets having a thickness of several millimeters. On the otherhand, the manufacture of glass sheets of small thickness (less than 3mm) by this process poses problems either in terms of efficiency inlarge plants or in terms of flatness and optical quality of the glass insmall plants.

According to another process forming the subject of patent FR 1 378 839and of its additions, numbers 86/221, 86/222, 86/817, 87/798 and 91/543,of patent FR 2 123 096 and patent FR 2 150 249, use is made of moveableguiding elements, which are continuous and flexible (for example metalwires) and adhere to the glass along the lateral edges of the ribbon andaccompany said ribbon in its movement over the bath. For smallthicknesses, this process does not allow satisfactory quality to beautomatically obtained either, particularly in small plants, the glasssheets obtained not always having a constant thickness and/or asatisfactory flatness.

The object of the present invention is to provide a process formanufacturing float glass that does not have the aforementioneddrawbacks, in particular making it possible to improve the flatness ofvery thin float glass produced in small plants (i.e. plants whosedimensions generally do not exceed 20 m in length and 4 m in width andwhich produce less than 20 tons of flat glass per day, large plantsbeing more voluminous and able to produce up to several hundred tons offlat glass per day). The present invention thus provides a process formanufacturing float glass tailored to the manufacture of thin glasssheets by the float technique and making it possible to obtain thinglass sheets exhibiting, in particular, good flatness, satisfactoryoptical quality and constant thickness, this process avoiding anydeformation of the sheet during the forming and also making it possibleto anneal the thin ribbon without creating residual form stresses whichalso run the risk of impairing the flatness of the sheet.

In the process according to the invention, molten glass is poured onto aliquid support denser than the glass, especially a bath of metal (forexample molten tin or a tin alloy), and then the continuous ribbon whichforms is advanced toward the downstream end, this process beingcharacterized in that the thickened edges of the ribbon are trimmed (orcut) continuously in the forming zone.

The expression “thickened edges of the ribbon” is understood to mean inmost cases the lateral edges of the ribbon, which have a substantiallygreater thickness than the central part of the ribbon, in particular athickness at least twice than that of the middle of the ribbon. Moregenerally according to the present invention, this expression alsocovers the edges having a substantially smaller thickness than thecentral part of the ribbon, such as a thickness half that of the middleof the ribbon or less.

These thickened edges generally occupy between 4 and 30% of the width ofthe ribbon. They are due to the float process and appear during theforming, it being possible for the addition of thickness to be greateror smaller depending on the process employed and on the type of productthat it is desired to obtain. In the case of a glass ribbon having athickness of the order of one millimeter or less at the center, thesethickened edges may have for example a thickness of about 7 millimeters.Trimming these edges, which have a thickness and therefore a temperatureand a stiffness that are different from those of the center of theribbon, makes it possible to limit the deformation of the ribbon due tothe effect of transverse forces and also makes it possible to preventthe appearance of residual form stresses in the ribbon, these residualstresses running the risk of impairing the flatness of the sheet duringits annealing in the lehr after the bath or running the risk ofembrittling the sheets and fracturing them. Instead of obtainingthickness differences of a few millimeters from one edge of the ribbonto the other, differences of a few hundredths of millimeters are thusobtained. This trimming is particularly important for small float plantsor for the manufacture of thin sheets where flatness is essential. Thistrimming also makes it possible, where appropriate, to limit the neckingphenomena observed after the lateral stretching elements and makes itpossible to reduce the heating means at the exit of the bath, the ribbonbeing heated more uniformly and rapidly.

The trimming mentioned in the definition of the invention should not beconfused with the cutting, already known, which is carried out after theforming zone. The purpose of the latter cutting, carried out on theglass that has set sufficiently to be able to be handled and/or on theannealed glass, is to give the product its final shape and dimensions,and consists in cutting the glass ribbon into sheets or in cutting aparticular shape in the ribbon or the sheets. Of course, this kind ofcutting to shape has nothing to do with the trimming according to thepresent invention which is carried out much further upstream andconstitutes a kind of precutting whose function is not to replace thecutting to shape, but, on the contrary, to fulfill a very particularfunction not obtained by the downstream cutting to shape. Moreover, thetrimming mentioned in the definition of the invention does not excludethe cutting to shape (nor does it dispense with this cutting) carriedout after the bath, the latter cutting possibly always taking place,where appropriate, in order to give the product its final shape anddimensions.

The trimming according to the invention is therefore carried out duringthe forming, in the forming zone or working zone (also called “zone 2”)corresponding to the zone where it is possible to work the product inorder to give it its final thickness, the ribbon then generally having aviscosity of between 10⁴ and 10¹⁰ poise, and preferably between 10⁴ and10^(7.6) poise. The temperature of the ribbon in the forming zone isusually between the point called the working point and the Littletonpoint, i.e. generally between 650° C. (or 700–740° C. depending on theglass) and 1150° C. In other words, the trimming according to theinvention is carried out, preferably and advantageously according to thepresent invention, at a temperature above the Littleton point of theglass used in the float.

In practice and preferably, the edges are trimmed between the momentwhen (or the point where) the glass ribbon reaches its maximum width inthe float (i.e. level with the final lateral stretching elements beforeany necking) and the moment when (or the point where) the glass ribbonseparates from the bath (i.e. generally the moment when it starts to belifted onto the lift-out rolls (LORs) placed at the exit of the bath,the point also being called the washline). Preferably and in most cases,they are trimmed just after the ribbon has reached its maximum width inthe float. In particular, the edges of the ribbon are generally trimmedat a ribbon temperature of about 900 to 1100° C., right in the formingzone, or else in a zone where the glass has a viscosity of between 10⁴and 10^(5.5) poise, well above the softening point of the glass, it beenbetter for the edges of the sheet to be separated when the glass is at atemperature well above its softening temperature, trimming the glass ata temperature close to its softening point running the risk of formingwaves in the ribbon. The edges are thus trimmed hot, in the still liquidglass, and without creating stresses. The edges are trimmedcontinuously, the trimming taking place along each of the sides of theribbon a few centimeters (for example 5–6 cm) from the edge of theribbon, the trimmed edges forming continuous strips.

The trimming is generally a physical or mechanical operation, and is forexample advantageously carried out by means of at least one laser and/orat least one hot knife.

In the laser case, it is advantageous to use a laser (such as a CO₂laser) emitting in the infrared, at a wavelength where the glass isopaque and consequently absorbs a maximum amount of light. Moregenerally, the laser must exhibit characteristics (such as thewavelength and the power or intensity) such that the glass can absorbthe energy of the laser beam that it receives and can convert it intosufficient heat to be given off at the point of impact, particularlyinto the liquid or vapor state. The power of the laser may, for example,be about 200 to 500 W, the efficiency generally increasing with thepower. Likewise, the efficiency increases when the speed of the ribbondecreases. Thus, it is possible to raise the glass locally totemperatures above 3000° C. for ribbon speeds of, for example, about 0.5to 1.5 m/min.

In general, the laser beam, emitted by a suitable device placed outsidethe float, is directed toward the point of impact in the float by asystem of lenses, mirrors and windows through an optical pipe whichpenetrates the float chamber (for example via a lateral port of thefloat). Advantageously, the optical part is installed in an inner armwhich can be easily removed, while leaving the outer pipe in the float,the pipe being cooled, for example by circulating water. The parts ofthe optical pipe which receive the laser beam must exhibit good infraredlight transmission properties and if necessary can be cleaned, forexample using nitrogen, by one or more nozzles (particularly for thebeam exit window located in the float).

Preferably, at least two lasers as described above are used, the lasersbeing placed on each side of the ribbon. Each beam is introducedrespectively via each pipe into a port on the side of the float and isdeflected so as to strike the glass ribbon at the trimming pointperpendicularly. The trimming takes place in the moving ribbon, thelasers advantageously remaining stationary.

In the knife case, it is advantageous to use a knife made of one or morematerials which can be heated by resistance heating (Joule effect) andwhich at the same time withstand the metal bath and the glass. Forexample, the knife is made of graphite, molybdenum, tungsten, dopedaluminum nitride, doped SI₃N₄, for example TiN-doped Si₃N₄,SiC/AlN/MoSi₂, etc. Preferably, a material is suitable if it withstandsat least 40 hours in the float bath. The hot knife is, for example, inthe form of an angular or bent piece (for example in the form of a U)through which current flows so as to heat a point or that part of thebend or of the angle having to cut the glass, this piece possibly havingone or more colder or cooled parts. The tip of the knife is heated to ahigh temperature, notably to temperatures possibly up to approximately1400–1500° C. (or even greater than 1500° C.), the temperature of theknife having to exceed the temperature of the glass by at least 300° C.The knife is preferably thin, for example with a thickness of less than2 mm (in particular at the point where its cross section is smallest) inorder to avoid causing instability phenomena such as, for example, wavesin the glass. Likewise, the speed of the ribbon when using a knife ispreferably low (especially less than 10 m/min) so as to prevent theformation of waves. The current flows in the lower part of the knifewhere the hot spot at the minimum cross section is produced. The knifeis placed in the glass ribbon and supplied via a pipe which also holdsit. The trimming is carried out in the moving ribbon, the knifeadvantageously remaining stationary.

Preferably, two knives as described above are used, for example twotungsten knives, the knives being placed on each side of the ribbon.

Of course, it is possible to combine knife trimming with laser trimming,the knife then being preferably placed just behind the laser in the slitcreated by the latter.

According to one embodiment of the invention (particularly when using alaser), a jet of gas may be directed toward the trimming point at thesame time as the trimming is being carried out. This jet of gas can helpto expel, where appropriate, the material heated and/or vaporized by thetrimming instrument, away from the trimming line, and can be used tocool the surfaces created in order to prevent the edges from rebonding.In general, the material is expelled toward the top either in theupstream direction or in the downstream direction of the float.Preferably in the case of a laser, the jet of gas is directed toward thepoint of impact of the laser on the glass or very close to this point ofimpact, whereas in the case of a knife the jet may be directed into theslit slightly downstream of the point of contact between the knife andthe glass. The jet of gas may be transverse to the movement of the glassor in the direction of movement of the glass ribbon (notably when thenozzle is placed upstream of the impact) or in the opposite direction.The flow rate of the jet may, for example, be about 2 to 19 l/min. Thisjet may advantageously be supplied via a tube inside the pipe used forsupplying the trimming device. Thus, when the pipe is cooled, the gasremains cold. The gas used may be nitrogen for example.

Advantageously, it is also possible (especially when using a laser) tolift the sheet at the trimming point using at least one generallyinclined piece referred to hereinafter as a “shoe”, for example agraphite shoe, so as to break the contact between the glass and themetal bath and to facilitate the trimming. The lift, by creating a spacebetween the sheet and the metal bath, may also allow the vaporized orliquid glass to be expelled downward and allow better separation of theedges by creating a local shear. Optionally, the trimming may be done atthe point where the ribbon starts to be lifted off the bath at the exitof the float so as to use this already existing lift rather thaninstalling one or more shoes. The shoe or shoes are preferablywater-cooled so as not to stick to the glass and are advantageouslyplaced downstream of the point of impact of the trimming device, closeto this point (for example, 5 mm from the point of impact) so that thecutting takes place when the ribbon starts to lift.

In one embodiment of the present invention, the glass ribbon isstretched laterally over the surface of the bath, in the forming zone,by gripping it laterally by top rolls (usually made of steel) which arepositioned along the bath on each side of the glass ribbon. In thiscase, the trimming instrument or instruments are preferably placed afterthe last top roll. The top rolls generally act at a viscosity of between10^(4.5) and 10^(5.5) and hold the ribbon to a set width. Together withthe longitudinal stretching imposed by the lift-out rolls at the exit ofthe float, the ribbon is stretched through a well-defined width and awell-defined thickness.

In a preferred embodiment of the present invention, the glass ribbon isstretched laterally over the surface of the bath, in the forming zone,and it is accompanied in its movement by means of continuous andflexible guiding elements made of a solid material capable of adheringto the molten glass, these elements being brought into contact with thelateral edges of the ribbon just behind the spout and then spreading outthe ribbon by means of two spreader fingers placed in a zone where theglass has a viscosity of between 5000 and 50 000 poise, these spreaderfingers forcing the guiding elements to diverge in order to carry outthe gradual transverse stretching of the ribbon to its final width andthickness, the separation between the guiding elements then beingmaintained until the glass has cooled. In this embodiment, the trimminginstrument or instruments are preferably placed just after the spreaderfingers, close to the spreader fingers.

In general, whatever the embodiment, the edges are preferably trimmed atthe final lateral stretching elements, particularly just after theseelements, the trimming device (such as one or more lasers and/or one ormore knives) then been placed after the last action by the lateralstretching elements (top rolls or spreader fingers).

In general too, the speed of the ribbon in the float is kept to lessthan 15 m/min, preferably less than 10 m/min, particularly preferablyless than 5 m/min and may be down to 150 mm/min for example (the speedof advance of the ribbon may, for example, be about 250–300 mm/min for afinal sheet thickness of about 1 mm). The term “final thickness” of theribbon is understood to mean the (average) thickness of the ribbon afterthe guiding elements have been detached or else the thickness of theribbon set by cooling, this thickness possibly going below 0.4 mm.

In the embodiment using continuous and flexible guiding elements, theproduct (P) of the value (v) of the ribbon speed (measured at the exitof the bath) and of the value (e) of its final thickness is generallyless than 2.5×10⁻³ m²/s, or even 2.5×10⁻⁴ m²/s or 1×10⁻⁴ m²/s (inpractice, it can go as low as 1×10⁻⁵ m²/s, or even lower).

The guiding elements and the spreader fingers (or retention pieces)mentioned above are especially described in patent FR 1 378 839 and itsadditions, in patent FR 2 123 096, in patent FR 2 150 249 and in patentFR 2 747 119 to which reference may be made for more information aboutthese guiding elements and their advantages. The guiding elementsaccording to the invention may, for example, be standard-type metalwires, notably wires made of stainless steel, (such as wires made ofbright annealed or black annealed mild steel), or made of a refractoryalloy, having a diameter of approximately 1 to 2 mm for example.

These guiding elements are generally stretched between devices such asreels located upstream and downstream of the bath and are placed incontact with the glass ribbon by means of bearing pieces or “pads” (inthe form of press wheels, rollers, bars, etc.) or by means ofintroduction ducts also described in the aforementioned patents. Thebearing pieces and the spreader fingers are produced so as to resistwear, due to the friction of the guiding elements and to corrosion bythe molten glass, and may also be heated. The guiding elements are leftin the edges of the glass or can be detached from the glass when thelatter has reached a stage in the process when its cooling has made itsufficiently rigid. If the wires remain inserted in the edges after thespreader fingers, they can also serve for pulling away the edges of theglass after trimming.

In the embodiment using guiding elements, the angle of divergence (i.e.the angle measured between the axis of the ribbon and the straight linepassing between a bearing piece and the closest retention piece lying onthe same side of the ribbon) is preferably chosen to be less, inabsolute value, than 25° and preferably less than 20° so as to help toimprove the quality and the flatness of the glass sheets obtained, agreater angle possibly causing the glass ribbon to be stretched tooabruptly.

The embodiment using the guiding elements is more economic and easier toemploy than the process using top rolls within the context of thepresent invention. The lifting using one or more of the abovementionedshoes is, in particular, made simpler and more reliable than in theembodiment using top rolls.

On leaving the bath, whatever the embodiment, the ribbon, which hasbecome sufficiently rigid, may be withdrawn from the bath by mechanicalmeans without suffering damage liable to affect its flatness and itssurface polish. It may be or have been annealed (for example in anannealing lehr) and/or subjected to other treatments. Next, it may becut (possibly by laser) into sheets and/or the ribbon or the sheets maybe cut to the required dimensions. Before cutting to the final shape,the product generally has a particular structure in that the lateraledges remaining after the thickened edges have been hot trimmed areslightly rounded or have a slight bead (this bead having, for example, athickness of a few tens of microns more than the central thickness ofthe ribbon) and possibly a slight rib before the rounded edge.

In a novel and advantageous manner, the ribbon may be wound in line(when its thickness does not exceed 500 or 700 μm, it being possible toachieve such thicknesses by the process according to the invention, asexplained below), something which hitherto has not been possible withthe presence of the thickened edges.

The present invention also relates to the rolls of film glass (i.e.glass with a thickness of less than 0.7 mm and preferably less than 0.4mm) conceived within the present invention and able to be obtained bythe process according to the invention. These rolls have advantages,especially in terms of handling and transportation, in addition to theadvantages due to the film glass. Preferably, to avoid fatigue, damageand production problems, such rolls according to the invention have aratio of the (internal) radius of the wound package to the thickness ofthe glass (the values being expressed in SI units) of greater than 1000,preferably greater than 2500 or 3500, or possibly even greater than 10000. Also to avoid damage problems, suitable inserts may be placedbetween the turns of the wound package.

According to one advantageous embodiment of the invention, the ribbonmay undergo a toughening operation; in particular, the edges of theribbon, which have possibly been weakened by the trimming, may bechemically toughened, either in line (for example, before the sheets arecut to the required dimensions or before the ribbon is wound) or afterthe ribbon has been wound. The advantage of carrying out the tougheningon the glass roll or wound package according to the invention is, inparticular, the fact that the toughening may be carried out more easilythan on the ribbon in line and at the same time over a great length ofglass ribbon. The edges of the ribbon are, for example, toughened bydipping the lateral parts of the wound package into a suitable bath (forexample in a tank of molten salt, such as a potassium salt) for the timeneeded to perform the toughening (and the possible healing) of thoseparts (this time being, for example, about one day or more).

As mentioned above, the invention applies particularly well to themanufacture of thin glass sheets having, for example, a thickness ofless than 2.5 mm and in particular a thickness of less than 0.7 or 0.4mm (film glass). There are many applications of the film glass obtainedaccording to the invention and possibly presented in the form of rolls;in particular, the film glass according to the invention can be used asprotection (protection of food, protection from scratches, electricalprotection, protection for providing impermeability to gases, liquids,chemicals, etc.), as decoration (after treatment, such as screenprinting, sputtering, etc., in order to deposit layers such as a mirror,enamel, etc.), the glass being in this case applied, for example, byunwinding a roll according to the invention against a surface which itis desired to decorate, as a carrier or promoter of a function (anoptical function, a chemical function, an adhesion-promoting function,an anti-fouling or anti-graffiti function, an anti-rain function, ananti-bacterial function, etc.), as an identifier or seal (for example asa watermark incorporated into banknotes and coated with a layer allowingthe banknote to be identified, or as a seal which is incorporated into abottle and which breaks when it is opened in order to authenticate theoriginal contents of the bottle, or as an insert placed in a card andmaking the latter unfalsifiable, falsification of the card resulting inthe glass breaking, etc.), as packaging, as a storage medium for opticalor magnetic data, as a screen, etc.

In the process according to the invention, flat glass may beadvantageously manufactured from all the compositions normally used infloat processes (conventional silica-soda-lime glasses). In theembodiment using guiding elements, it is also possible to use glasseshaving a working range (the difference between the temperature at whichthe glass has a viscosity of 1000 poise and the temperature at which theglass has a viscosity of 10⁶ poise) which is shorter, especially lessthan 350° C. and particularly less than 300° C., the process in thisembodiment making it possible to form the glass to its final thicknessmore rapidly.

Glasses that can be used advantageously in the process according to theinvention are, for example, the glasses described in patents and patentapplications EP 526 272, EP 576 362, FR 2 727 399, FR 2 725 713, FR 2725 714, WO-96/11888 and WO-96/11887.

The products obtained according to the process of the invention exhibitgood flatness (the camber over a length of about 50 cm not generallyexceeding 100 μm) and a relatively constant thickness. The opticalquality of the glasses obtained is also at least equal to or evensuperior to that of the glasses obtained according to the currentprocesses. As indicated above, the necking of the glass ribbon is veryslight and does not result in optical defects. Nor does the ribbonobtained present difficulties in annealing. The process according to theinvention is also economic and particularly efficient.

The present invention also relates to a plant (float furnace or float)suitable for implementing the process according to the invention, thisplant comprising at least one trimming device (for example using one ormore lasers and/or one or more knives) preferably placed after the finalaction by the lateral stretching elements (top rolls or spreaderfingers). The plants according to the invention, already explained to agreat extent in the above description of the invention, are alsoillustrated below, through a nonlimiting example with reference to theappended FIGS. 1 and 2 which show:

FIG. 1: a partial schematic view from above of a plant according to theinvention;

FIG. 2: a partial schematic section on the line A—A of the plant in FIG.1.

The plants according to the invention are preferably short low-output(less than 20 t/d) plants.

The plant generally comprises a tank, part of the enclosure of which isillustrated at 1, this tank containing a metal bath 2, such as a tinbath for example. The upstream part of this tank may have divergingwalls or “restrictors” 3 in the direction of advance of the glass. Themeans for heating the tank have not been shown.

The molten glass is fed at the upstream end of the tank into the zonedepicted at 4. The glass thus poured in spreads out over the bathbetween the diverging walls under the action of gravity and of thepulling exerted downstream. Wires 5 (the guiding elements), coming forexample from reels 6, are then introduced via the upper face into theedges of the sheet before the latter has left the diverging walls, orimmediately after (or even at the spout 4). Each wire may be fed via awire guide or duct 7 which may possibly impose a change of directionthereon, this arrangement making it possible, where appropriate, toplace the reels laterally with respect to the furnace and to feed thewire in the desired position, and then each wire encounters a bearingpiece 8 which subjects it to a downwardly directed force in order toinsert it into the glass.

Downstream of the spreading-out zone there are spreader fingers 9 whichforce the wires to diverge and then prevent them from coming closertogether due to the action of surface tension forces. The wires bear onthese spreader fingers and keep the ribbon 20 (shown only in FIG. 2) tothe width corresponding to the final thickness to be obtained. The wiresaccompany the ribbon while it cools down in the downstream zone of thebath (not shown) until the ribbon is sufficiently rigid (the ribbontemperature being, for example, less than 600° C.).

Just after the spreader fingers, the thickened edges of the glass ribbonare trimmed using CO₂ lasers 11. The laser beam 14 emanating from a CO₂laser 11 placed outside the float, along each side of the float, is sentvia mirrors 17, 18, lens 19 and windows 15, 16 along an optical pipe 13penetrating the float via a lateral (side-sealing) port 22. The opticalpipe is cooled and rests on an optical bench 12, the whole assemblybeing placed on a raiseable table 21 in order to adjust the position ofthe trimming system.

The process according to the invention makes it possible to manufacturethin glass sheets that can be used especially in the automobile field,the electronics field, in plasma screens, magnetic hard disks, etc.

1. A float process for manufacturing glass sheets comprising: pouringmolten glass onto a liquid support denser than the glass; forming acontinuous glass ribbon from the molten glass; advancing the continuousglass ribbon toward a downstream end; and continuously trimmingthickened lateral edges of the glass ribbon in a forming zone at atemperature well above the softening point of the glass while saidthickened lateral edges are in contact with said liquid support.
 2. Theprocess as claimed in claim 1, wherein the lateral edges are trimmedbetween a moment when the glass ribbon reaches its maximum width and amoment when the glass ribbon separates from the liquid support.
 3. Theprocess as claimed in claim 1, wherein the lateral edges are trimmed ata temperature above a Littleton point of the glass.
 4. The process asclaimed in claim 1, wherein the lateral edges are trimmed with at leastone laser and/or at least one hot knife.
 5. The process as claimed inclaim 1, further comprising: directing a jet of gas toward a trimmingpoint at a same time as trimming is being carried out.
 6. The process asclaimed in claim 1, further comprising: stretching the glass ribbonlaterally over a surface of the liquid support in the forming zone;providing continuous and flexible guiding elements made of a solidmaterial capable of adhering to and moving with the glass ribbon;spreading out the ribbon using two spreader fingers; and performing thetrimming step using a trimming instrument or instruments placed justafter the spreader fingers.
 7. The process as claimed in claim 1,wherein a speed of the ribbon is less than 10 m/min.
 8. The process asclaimed in claim 1, further comprising: chemically toughening thelateral edges of the ribbon.
 9. The process as claimed in claim 1,wherein said temperature is at least 900 degrees Celsius.
 10. Theprocess as claimed in claim 1, wherein the glass ribbon obtained forms afilm of low thickness or is cut into sheets.
 11. A device comprising: aliquid support denser than glass; a glass pouring unit configured topour molten glass onto said liquid support; a glass ribbon forming unitconfigured to form a continuous glass ribbon from the molten glass; andat least one trimming device placed in the forming zone configured tocontinuously trimming thickened lateral edges of the glass ribbon at atemperature above the softening point of the glass while said thickenedlateral edges are in contact with said liquid support.
 12. The device asclaimed in claim 11, wherein said device has dimensions not exceeding 20m in length and 4 m in width and produces less than 20 tons of flatglass per day.
 13. A float process for manufacturing glass sheetscomprising: pouring molten glass onto a liquid support denser than theglass; forming a continuous glass ribbon from the molten glass;advancing the continuous glass ribbon toward a downstream end; andcontinuously trimming thickened lateral edges of the glass ribbon in azone where the glass has a viscosity of between 10⁴ and 10^(5.5) poise.14. A float process for manufacturing glass sheets comprising: pouringmolten glass onto a liquid support denser than the glass; forming acontinuous glass ribbon from the molten glass; advancing the continuousglass ribbon toward a downstream end; and continuously trimmingthickened lateral edges of the glass ribbon just after the ribbonreaches its maximum width while said thickened lateral edges are incontact with said liquid support.
 15. The process as claimed in any oneof claims 1, 13, or 14 further comprising: winding the glass ribbon toform a roll of glass.
 16. A float process for manufacturing glass sheetscomprising: pouring molten glass onto a liquid support denser than theglass; forming a continuous glass ribbon from the molten glass;advancing the continuous glass ribbon toward a downstream end;continuously trimming thickened lateral edges of the glass ribbon in aforming zone at a temperature well above the softening point of theglass; directing a jet of gas toward a trimming point at a same time astrimming is being carried out.
 17. A float process for manufacturingglass sheets comprising: pouring molten glass onto a liquid supportdenser than the glass; forming a continuous glass ribbon from the moltenglass; advancing the continuous glass ribbon toward a downstream end;lifting the glass ribbon at a point where the trimming step is performedso as to break contact between the glass ribbon and the liquid support;and continuously trimming thickened lateral edges of the glass ribbon ina forming zone at a temperature well above the softening point of theglass.
 18. A float process for manufacturing glass sheets comprising:pouring molten glass onto a liquid support denser than the glass;forming a continuous glass ribbon from the molten glass; advancing thecontinuous glass ribbon toward a downstream end; stretching the glassribbon laterally over a surface of the liquid support in the formingzone; providing continuous and flexible guiding elements made of a solidmaterial capable of adhering to and moving with the glass ribbon;spreading out the ribbon using two spreader fingers; and continuouslytrimming thickened lateral edges of the glass ribbon in a forming zoneat a temperature well above the softening point of the glass using atrimming instrument or instruments placed just after the spreaderfingers.
 19. A device comprising: a liquid support denser than glass; aglass pouring unit configured to pour molten glass onto said liquidsupport; a glass ribbon forming unit configured to form a continuousglass ribbon from the molten glass; and at least one trimming deviceplaced in the forming zone configured to continuously trimming thickenedlateral edges of the glass ribbon at a temperature above the softeningpoint of the glass while said thickened lateral edges are lifted so asto break contact between the glass ribbon and the liquid support.